French Patent Application No. 2559008 filed by Sony, Jan. 31, 1985, describes a cordless telephone set comprising a base unit and a mobile unit having each an identification code: for a communication to be set up between the base unit and the mobile unit, their identification codes are to match. This patent application describes a mechanism for matching said identification codes by radio channel which requires that the mobile unit is put back on the base unit.
Such a solution has various drawbacks. On the one hand, it necessitates the use of physical detection means for detecting that the mobile unit rests on the base unit, which considerably increases the cost price of the telephone. On the other hand, it does not permit of changing the identification code at an arbitrary instant: the only criterions applicable are of the type "change of identification codes every n time intervals in which the mobile unit rests on the base unit". Finally, this process does not provide total reliability for the user, notably in the case where the user lifts the mobile unit before the base unit has received a clearing from the mobile unit.